LAKES 335 



Many lakes, some of them large * and many of them small, are 

 known to have become extinct, while many others are now in their 

 last stages, namely, marshes. Many others have been greatly 

 reduced in size. Such reductions are often obvious where deltas 

 are built into lakes. Thus the delta built by the Rhone into Lake 

 Geneva is several miles in length, and has been lengthened nearly 

 two miles since the time of the Roman occupation. The end of 

 Seneca (N. Y.) Lake (PL XXI) has been crowded northward some 



Fig. 270. Terraces on the shore of the ancient Lake Lahontan, north of 

 Pyramid Lake, Nevada. (Fairbanks.) 



two miles by deposition at its head. Similar changes have taken 

 place and are now in progress in many other lakes. 



Salt lakes. A few lakes, especially in arid or semi-arid regions, 

 are salt, and others are " bitter." Beside common salt, salt lakes 

 usually contain magnesium chloride, and magnesium and calcium 

 sulphates, as well as some other mineral substances. "Bitter" 

 lakes usually contain sodium carbonate, as well as sodium chloride 

 and sulphate, and sometimes borax. The degrees of saltness and 

 bitterness vary from freshness on the one hand to saturation on 

 the other. The water of the Caspian Sea flake) contains, on the 



1 Gilbert, Lake Bonne ville, Mono. I, U. S. Geol. Surv.; Russell, Lake 

 Lahontan, Mono. XI, U. S. Geol. Surv.; and Mono Lake, Eighth Ann. Kept., 

 U. S. Geol. Surv., Pt. I; Upham, Lake Agassiz, Mono. XXV, U. S. Geol. 

 Surv.; Salisbury and Kiimmel, Lake Passaic, Kept, of the State Geologist 

 of N. J., 1893, and Jour, of Geol., Vol. Ill, pp. 533-560. 



